The present invention relates to a thermal ink printer and, more particularly, to a novel printhead which extends the printing range perpendicular to the process direction by selective shifting of the printhead. In existing thermal ink jet printing, the printhead typically comprises one or more ink ejectors, such as disclosed in U.S. Pat. No. 4,463,359, each ejector including a channel communicating with an ink supply chamber, or manifold, at one end and having an opening at the opposite end, referred to as a nozzle. A thermal energy generator, usually a resistor, is located in each of the channels a predetermined distance from the nozzles. The resistors are individually addressed with a current pulse to momentarily vaporize the ink and form a bubble which expels an ink droplet. As the bubble grows, the ink rapidly bulges from the nozzle and is momentarily contained by the surface tension of the ink as a meniscus. As the bubble begins to collapse, the ink still in the channel between the nozzle and bubble starts to move towards the collapsing bubble, causing a volumetric contraction of the ink at the nozzle and resulting in the separation of the bulging ink as a droplet. The acceleration of the ink out of the nozzle while the bubble is growing provides the momentum and velocity of the droplet in a substantially straight line direction towards a print sheet, such as a piece of paper. Because the droplet of ink is emitted only when the resistor is actuated, this type of thermal ink-jet printing is known as "drop-on-demand" printing. Other types of ink-jet printing, such as continuous-stream or acoustic, are also known.
In a single-color ink jet printing apparatus, the printhead typically comprises a linear array of ejectors, and the printhead is moved relative to the surface of the print sheet, either by moving the print sheet relative to a stationary printhead, or vice-versa, or both. In some types of apparatus, a relatively small printhead moves in the process direction across a print sheet numerous times in swaths, much like a typewriter; alternatively, a printhead, which consists of an array of ejectors extending the full width of the print sheet, is incorporated into what is known as a "full-width array" (FWA) printer. When the printhead and the print sheet are moved relative to each other, imagewise digital data is used to selectively activate the thermal energy generators in the printhead over time so that the desired image will be created on the print sheet.
With ink-jet printing, it is also possible to create multicolor images on a print sheet. This type of printing may be used for full-color images, such as to reproduce a color photograph, or can be employed for "highlight" color, in which colored additions are made to a main portion of the image or text, which is typically black. In either case, the most common technique for color ink jet printing has been to sequentially image two or more colors, in separate printing steps, onto the single print sheet. This superimposition can be carried out in any number of ways. For example, a single printhead may be segmented with different colinear sections of the printhead dedicated to different colors, so that the different colors are printed in subsequent passes, with a paper advance between passes. Alternately, two or more printheads may be positioned very close and substantially parallel to each other, and render the two or more portions of the image onto the print sheet almost simultaneously, although different areas of the print sheet will be printed upon by the different printheads at the same time or with a small time lag. For a full-color process image, four types of ink (yellow, magenta, cyan, and black) may be emitted from four separate printheads during printing as the print sheet is moved relative to them.
The above black and color printers are designed to accommodate a print zone having the same width as the length of the printhead; e.g., a printhead with 64 jets, 128 jets, 256 jets, etc. For many machines, a relatively small print zone (swath) is used in order to maintain a small gap (typically 1 mm or less) between the nozzle surface and the print sheet. Control of such a gap is most easily achieved by limiting the size of the print zone. This results in various use restrictions. For example, if a user wishes to change the drop size characteristics of a black only printhead to achieve gray scale printing, or different optical density for different media (such as paper versus transparencies), a different black printhead with the appropriate drop size must be substituted or added as a second printhead. In the earlier cited example of a segmented colinear color printhead, the printing throughput is reduced relative to a monochrome printhead of the same size, because fewer jets are available for each color. It would be advantageous for these and for other printer applications discussed in further detail below to have both color and black printing capabilities, or different drop size printing capabilities, coresident in the printer without a throughput loss and using only a relatively small print zone for printing, and further without the need for an expensive machine having a larger, or containing multiple, printheads.